Usually, an unsaturated polyester resin is produced by dissolving an unsaturated polyester in at least one monomer selected from vinyl monomers, allyl monomers and other copolymerizable monomers, and when required, may be added with various additives such as, promoters, inhibitors, accelerators, thixotropic agents, thixotropic auxiliary agents, UV absorbers, fire retarding agents, pigments, waxes, or the like.
An unsaturated polyester which is one of the main two components of an unsaturated polyester resin is generally obtained from a saturated dibasic acid or its anhydride or its diester, an unsaturated dibasic acid or its anhydride and a glycol or an alkylene oxide or from an unsaturated dibasic acid or its anhydride and a glycol or an alkylene oxide.
It is also known that an unsaturated polyester is obtained from a depolycondensation product prepared from a high molecular weight saturated polyester and other raw materials required for the formation of an unsaturated polyester.
In the latter case, a polyethylene terephthalate for uses in fibers, films or molding materials or a large amount of polyethylene terephthalate wastes by-produced during the manufacture or the fabrication of the said products, is utilized as the high molecular weight saturated polyester. In many patent specifications, there is disclosed that an unsaturated polyester resin is produced from an unsaturated polyester obtained from a depolycondensation product prepared from a polyethylene terephthalate, particularly for use in fibers, as the high molecular weight saturated polyester.
The polyethylene terephthalate for use in fibers generally contains titanium dioxide of from 0.1 to 2% by weight as a delustering agent. Since the unsaturated polyester resin thus obtained inherently contains titanium dioxide, the titanium dioxide contained therein will cohere or be sedimented within a few days due to the greater gravity of titanium dioxide than that of the unsaturated polyester resin, in many cases, into a formation of a cake. Such an unsaturated polyester resin is hardly suitable for commercial uses.